Straight from "The Man" himself!!

What to do if you miss a day or two.

Improvement and change occur when you do things often. Stopping and starting all the time will kill any momentum you need to succeed. You must find ways to stay in the game. Done consistently, moderate forms of exercise provide far better results than the occasional full-body pummeling. A lifestyle that includes doing multiple forms of exercise 5 to 6 days a week guarantees results.

[However...]

Stop beating yourself up if you can't sustain and/or maintain your "perfect" plan. It's okay to miss a workout once in a while. It doesn't mean that your process has gone to hell in a handcart. It doesn't mean you have to start over. Life happens. Priorities shift. So what? Big deal. Just start up where you left off. If you're doing Power 90 or P90X®, just add the missed days to the end of the program. I decree the burden lifted! Of course, you must recognize the difference between a missed workout or two and a missed week or two. If you miss 2 weeks of exercise, it will take at least that long to get back to where you left off. If you miss one workout once in a while, you lose nothing.